Been messing around in Slack for a couple of hours, so far found the transparent apps, the display of desktops contents (ie desktop 1-4), karm (time recording of sorts), the add "applet to panel"...ehh.. applet.
Konquerors metabar shows details about anything and actions for it. Kmail's sieve script management, although I haven't figured out what that is yet , a few imap tweaks.
Superkeramba, konqueror enhancements including basic add blocking and acid2 complience.

So far, nothing ground breaking from 3.4.3. But overall, some nice tidy ups, and so far, no gotchas.

It's still in the "experimental" repos at PCLinuxOS but I have played with it (I keep two parallel installs going and I only take risks on one!) and it seems very nice, though hardly the advance I was expecting.

If your distro uses GTK-QT engine, it cannot cope with KDE3.5 and Gnome>=2.8, so you have to uninstall it! Strangely enough, my Gnome apps have retained the Gtk-Qt settings, though I doubt that will remain the case if I change KDE style, it'll probably just stick where it is.

If you have selected a Mouse Pointer theme in KDE control centre, it does not use the ~/.icons/default folder to store the settings, but non-KDE apps will read from there, resulting in the cursors potentially changing when you use certain programs. I have corrected the ~/.icons/default/index.theme file, though I have it on good authority that deleting the folder altogether may also work.

Some people have experienced huge slowdowns, this is because SuperKaramba is included in KDE3.5, the best bet is not to run it unless your hardware can cope. One guy on the PCLOS mailing list found that one applet was tying up 82% of his CPU! But just because it's there, it's not compulsory!